Remore `restrict-eval`

[XYenon]

With restrict-eval, inputs other than <nixpkgs> are not allowed, severely limiting the NUR. For instance, it prevents using rust-overlay to build packages with nightly Rust. Additionally, I discovered that Nix plans to deprecate the restrict-eval option NixOS/nix#8295.

[Pandapip1]

If I'm not mistaken, the reason this is done is for flakes support. I'll investigate this later.

[XYenon]

I think it is to prevent token leaks #92, but pure eval should do the same thing.
https://github.com/NixOS/nix/blob/cacab33f0df21c2ea54d553d75872bcf50717269/src/libexpr/primops.cc#L985-L989
Does NUR allow impure?